Loss of fragile X mental retardation protein (FMRP) leads to fragile X syndrome, the most common form of inherited mental retardation, which is associated with cognitive impairment and more numerous and thinner dendritic spines in cortical neurons. FMRP is an mRNA binding protein and has been shown to stabilize select mRNA species and to repress local protein synthesis in dendrites. However, its role in dendritic targeting of mRNAs remains to be determined, although it is associated with transport RNA granules. Mice with deletion of the gene for FMRP, Fmr1, have deficits in hippocampus-dependent learning. In agreement with this phenotype, long-term potentiation (LTP) elicited by threshold levels of theta burst stimulation is impaired in hippocampal slices prepared from Fmr1 knockout (Fmr1 KO) mice, which can be rescued by application of brain-derived neurotrophic factor (BDNF). The Bdnf gene produces two pools of BDNF mRNA species, with either a short 3'untranslated region (3'UTR) or a long 3'UTR. The long 3'UTR has been shown to be necessary and sufficient to target BDNF mRNA to dendrites. Interestingly, a mouse mutant in which dendritic targeting of BDNF mRNA is impaired also exhibits more numerous dendritic spines and deficits in LTP at hippocampal CA1 synapses, as does the Fmr1 knockout. This research project will determine the role of FMRP in trafficking of BDNF mRNA to dendrites. In Aim 1 the association of FMRP with BDNF mRNA will be examined in brain lysates using immunoprecipitation and in cultured neurons using in situ hybridization and immunocytochemistry. Electrophoretic mobility shift assays will be employed to identify FMRP binding sites in the long BDNF 3'UTR. In Aim 2 two studies will be carried out to determine whether FMRP is required for trafficking of BDNF mRNA to dendrites. One is to examine levels of BDNF mRNA in dendrites of Fmr1 KO neurons using in situ hybridization, and the other is to determine the effect of mutations in the FMRP binding sites on dendritic targeting of BDNF mRNA in cultured neurons. In Aim 3 the role of FMRP in the local synthesis of the BDNF protein will be examined in cultured neurons and synaptoneurosome preparations. PUBLIC HEALTH RELEVANCE: Fragile X syndrome is the most common form of inherited mental retardation, which is associated with a wide range of neurological symptoms and signs. This research will make a significant contribution to the understanding of fragile X syndrome.